The limestone caves at Carlsbad Cavern were formed by hydrogen-sulfide-rich (H2S) waters. This water moved through fractures and faults in the limestone and mixed with rainwater along the way. The mixture gave rise to a substance known as sulfuric acid (H2SO4). Sulfuric acid is able to dissolve limestone, eventually creating Carlsbad Cavern. The process of cave formation ended roughly four million years ago.